4.3.2.1. Hydrological Systems

Hydrological systems are potentially very sensitive to changes in climate.
The three key variables are soil moisture, which is a primary control on vegetation
and ecosystems; groundwater recharge, which feeds groundwater reserves; and
runoff, which feeds rivers and causes floods. Increased temperatures are expected
to cause increased potential evaporation and less snow, and possible changes
in mean rainfall, rainfall intensity, and rainfall seasonality would affect
soil moisture, streamflow, and groundwater recharge and the occurrence of floods
and droughts. More frequent high-intensity rainfall would tend to increase the
occurrence of flooding. Specific effects will depend on the pattern of change,
in rainfall particularly, and the characteristics of catchments and cannot be
predicted with confidence. In general, the drier the climate, the greater the
sensitivity to climate change (IPCC 1996, WG II, Chapter 10). Although the effects
of rainfall changes and sea-level rise on groundwater resources are not adequately
understood at present, they cannot be ignored (IPCC 1996, WG II, Section 10.3.6)
and may be significant for inland and coastal aquifers in Australia (Ghassemi
et al., 1991).

Water resources in the region are strongly affected by the heavy rainfall of
major weather events, such as tropical cyclones in northern Australia, and by
the ENSO phenomenon, which is the main source of year-to-year variation and
contributes both widespread heavy rainfall and widespread drought, depending
on its phase. However, climate models are unable to represent these well as
yet and therefore cannot represent the resulting major sources of surface runoff
and groundwater recharge events. This situation presents a basic difficulty
in assessing climate change impacts on hydrological systems and water resources.

A number of preliminary assessments and research studies of hydrological response
in the region are available (e.g., Griffiths, 1990; Mosley, 1990; Bates et al.,
1994; Chiew et al., 1995; Bates et al., 1996; Fitzharris and Garr, 1996; Minnery
and Smith, 1996; Schreider et al., 1996). Most of the research work has been
based on available regional scenarios of temperature and rainfall changes (see
Section 4.2.3) and has focused on changes in water yield
from unregulated rural catchments. Recent studies have begun to consider impacts
on groundwater recharge (Green et al., 1997) and on water resources systems
as a whole (Hassall and Associates, 1997; see Box 4-1).

A wide range of significant changes in water yield and soil moisture was found
by Chiew et al. (1995), who considered the potential impacts of CSIRO (1992)
climate scenarios on 28 catchments that represent the large range of climatic,
physical, and hydrological regimes experienced in Australia. By 2030, increases
in annual runoff of up to 25% and 10% occurred for catchments in the wet tropics
of northeastern Australia and in Tasmania, respectively. Decreases of up to
35% occurred for South Australia, and changes of ±20% and ± 50% occurred for
southeastern Australia and the west coast, respectively. Changes in annual soil
moisture levels ranged from -25% to +15%. Although the specific figures have
high uncertainty (arising from the large scenario uncertainty), their magnitudes
provide an indication of the size of changes that may conceivably occur.

Sizable changes in median monthly runoff during the wettest parts of the year
and increases in annual maximum monthly runoff were found by Bates et al. (1996),
who used the results of a single climate model (CSIRO9, experiment F1, Table
1-1) and a stochastic daily weather generator to represent climate and hydrological
variability under current and doubled CO2 conditions. The increases were due
to the general increase in rainfall intensity and the increased frequency of
heavy rainfall events indicated by the CSIRO9 model.

A similar shift toward greater variability was found by Schreider et al. (1996),
who applied the "most wet" and "most dry" climate change scenarios for 2030
and 2070 (adapted from CSIRO, 1992) to historical daily rainfall and temperature
series for 14 rivers in the Ovens and Goulburn Basins in southeastern Australia.
Figure 4-2 shows that in scenarios in which rainfalls
were projected to decrease ("most dry" scenario), the frequency of high-flow
events substantially decreased, and the drought frequency (as indicated by a
soil wetness index) increased. However, in scenarios in which rainfalls were
projected to increase, there was little increase in average annual runoff, but
the frequency of high flow events increased.

Figure 4-2: Change in frequencies of runoff and soil wetness for
different climate scenarios. White bars are for 2030, and black bars are
for 2070. Under the "most dry" scenario (top row), where rainfall decreases
and warmings are relatively large, the frequencies of high runoff and high
soil wetness substantially decrease. However, under the "most wet" scenario
(bottom row), where rainfall increases and warmings are less, there is some
increase in the highest runoffs but also some decreases in the highest soil
wetness frequencies. For further details, see Schreider et al. (1996).

Changes in catchment vegetation-either from climate change directly or from
adaptation responses (such as afforestation)-would alter catchment hydrological
characteristics, including evaporation, runoff, and extreme events (IPCC 1996,
WG II, Sections 14.2, 14.4). The effects of changes in rainfall amount and timing
on groundwater recharge can be amplified by the dynamic response of vegetation,
according to a study by Green et al. (1997) that used a daily soil-vegetation-atmosphere
model to determine changes in groundwater recharge beneath North Stradbroke
Island in northeastern Australia. It was found that the net recharge increased
consistently by amounts greater than the change in rainfall and that the recharge
was more affected by vegetation type than by soil type.

The responses to climate change of the large, arid, ephemeral lake systems
of interior Australia are difficult to predict. Significant water-level changes
may occur for nonephemeral lakes in dry evaporative drainages or small basins
where present evaporation is comparable with rainfall inputs (IPCC 1996, WG
II, Section 10.3).

Hydrological systems in the future also will be affected by other changes such
as deforestation and urbanization, both of which tend to increase runoff amount
and runoff speed-increasing the risks of flash flooding, high sediment loadings,
and pollution. Changes to water pricing and allocation mechanisms also will
affect patterns of water use and demand (Fenwick, 1995; McClintock, 1997), and
indeed can be used as adaptation measures.